1. Field of the Invention
The present invention relates to memory devices, for example, a resistance change memory device.
2. Description of the Related Art
Memory devices which use, as their storage elements, elements whose resistance varies in accordance with their states are known. Such variable resistance elements include magnetoresistive elements exhibiting the magnetoresistive effect. There is known a magnetic random access memory (MRAM) which comprises a memory cell array, in which the magnetoresistive effect elements are arranged in a matrix, and control circuits. The memory cell array and the control circuits enable access to an arbitrary bit in the MRAM.
The magnetoresistive effect includes the tunneling magnetoresistive (TMR) effect, and an element exhibiting the effect is widely used. Such a TMR effect element includes two layers of a ferromagnetic metal sandwiching an insulating film, and is typically implemented by means of a magnetic tunnel junction (MTJ) element which utilizes changes in magnetic resistance caused by the spin polarization tunnel effect. One of two magnetic films in which the orientation of magnetization is fixed is typically referred to as a “pinned layer” or the like. The other magnetic film in which the orientation of magnetization is programmable to be parallel or antiparallel to the pinned layer is referred to as a “free layer” or the like. Reading information held in an MTJ element is enabled by reading resistance determined in accordance with the relationship between the magnetization directions of the two magnetic films of the selected MTJ element by the use of voltage and current.
On the other hand, regarding writing, instead of a “magnetic field write” scheme which changes the magnetization direction of the free layer by applying a magnetic field thereto, a writing scheme referred to as a “spin-transfer torque writing” is proposed. This scheme involves using action of spin-polarized electrons to reverse the magnetization directly in order to perform data writing, as disclosed, for example, in the specification of U.S. Pat. No. 5,695,864. The spin-transfer torque writing scheme enables writing when a current with a density greater than or equal to a certain threshold flows in an element, and therefore can reduce the required current as miniaturization of the element progresses. The most significant issue of the spin-transfer torque writing scheme is reduction in write current density. There are attempts to design ideas about the material and the design of an element for the purpose of overcoming this issue. In addition, measures of designing ideas about circuitry to increase a current flowing in an element are also very important.
A memory cell typically includes one select transistor and one MTJ element which are connected in series. FIG. 66 and the corresponding description of Jpn. Pat. Appln. KOKAI Publication No. 2004-348934 discloses technology of increasing a current in an MTJ element. In the technology, two select transistors connected in parallel are connected in series to an MTJ element, and two cells adjacent to each other along the word line direction share a contact to a first bit line or a second bit line. Since a current is passed to an MTJ element of one memory cell by two select transistors, the current flowing through the MTJ element can be increased to be larger than with only one select transistor. The KOKAI Publication also achieves downsizing of a memory cell. However, the technology is based on a magnetic field write scheme, and therefore cannot be employed as it is for a spin-transfer torque writing scheme. In the spin-transfer torque writing scheme, since different information is written depending on the orientation of the current, bidirectional currents need to flow between the select transistor and the MTJ element. Accordingly, its requirements differ from those of the magnetic field write scheme.